There are many applications where it is desirable to control the amount of average electrical power delivered to a load. One example of such an application is the use of a lighting dimmer to control the output of a lamp. A dimmer typically functions by controlling the conduction of current through the load. A controllably conductive device is synchronized to the AC line voltage and is controlled to conduct for a predetermined interval in each half cycle of the AC line voltage. That is, the load only receives power (is on) for a portion of the AC line voltage half cycle. The longer is the conduction time, the more power that is delivered to the load. By the same logic, the shorter is the conduction time, the less power that is delivered to the load.
There are primarily two methods for controlling AC loads such as lighting loads: forward phase control and reverse phase control. A controllably conductive device is a device whose conduction can be controlled by an external signal. These include devices such as metal oxide semi-conductor field effect transistors (MOSFET), insulated gate bi-polar transistors (IGBT), bi-polar junction transistors (BJT), triacs, silicon controlled rectifiers (SCRs), relays, switches, vacuum tubes and the like. These two control methods utilize the conductive and non-conductive states of a controllably conductive device to control the power in a load and synchronize the conduction and non-conduction of the controllably conductive devices to zero crosses of the source of AC line voltage.
To energize the load, the method of forward phase control, as shown in FIG. 13, synchronizes a controllably conductive device to the source of AC power and controls the controllably conductive device to be non-conductive over the first portion of an AC line voltage half cycle, then controls the controllably conductive device to be conductive over the remaining portion of the AC line voltage half cycle. In the method of reverse phase control, as shown in FIG. 14, the periods of non-conduction and conduction are reversed with respect to time. That is to say, to energize the load, the controllably conductive device is controlled to be conductive during the first portion of the AC line voltage half cycle followed by a period of non-conduction in the same half cycle. The method of reverse phase control is often used for operation of capacitive loads such as electronic transformers.
In forward phase control based control systems the controllably conductive device is often a triac or an SCR. These devices can be controlled to be non-conductive or conductive. However, if they are controlled to be conductive, they can only be made non-conductive by allowing the current through them to go to zero. Due to this characteristic, these types of controllably conductive devices are not used for reverse phase control based control systems where the ability to enable and disable conduction is required.
Electronic controls need to derive a power supply in order to power their associated electronics. Additionally, many controls require line frequency related timing information. Controls which only have two power terminals have one of these terminals (the hot terminal) connected to a hot wire of a source of AC power and the other terminal (the dimmed hot terminal) connected to a first terminal of a load. Controls with this type of connection are often referred to as “two wire” controls. Two wire controls which are connected in series with their loads must charge their power supplies and obtain timing information through this load. The load can often have a wide range of input impedance. As such, the operation of the power supply and timing circuit is often compromised in the two wire connection scheme. However, a two wire connection is necessary when the control is wired in an application where a neutral wire is not available.
Controls which have connections to the hot wire, load, and neutral wire are often referred to as “three wire” controls. When a neutral wire from the source of AC power is available for connection to a neutral terminal of the control, the power supply and zero cross information can be derived independently of the connected load, thereby enhancing performance. In many applications, a neutral wire from the source of AC power is not available. Therefore, a control is needed that can operate correctly as either a two wire or three wire control, thereby allowing the control to be used in a broad range of field applications with great flexibility.
Prior art for developing a non-isolated low voltage power supply from a high voltage source, such as the AC line voltage, used circuits such as a cat ear power supply. Such a system would conduct at or near the line voltage zero cross so as to recharge an energy storage capacitor. Such systems typically operate properly in the region about 1 millisecond from the zero crossing of the line voltage. Operation outside that time window can cause excessive power to be dissipated in the power supply.
The cat ear power supply has relatively high peak and high average input currents with respect to the average current supplied to the connected DC load. This high average input current presents a significant problem when this supply technology is used with electronic low voltage (ELV) load types on phase control dimmers connected in a two wire mode. A supply for low voltage control circuitry is needed that has low average input currents through the high voltage load. Also, typical prior art power supplies have been relatively inefficient so that they require higher average input currents to supply the power requirements of typical prior art dimmers.
Another disadvantage of prior art power supplies for lighting control devices is that power losses in the power supplies increase with the amount of current required to be delivered by the power supply. The trend in modern lighting controls is to incorporate more features and functionality. These features and functionality require ever increasing amounts of current to be delivered by the power supply. Hence, it is desired to provide a power supply for a lighting control able to efficiently supply greater amounts of current than are presently available from typical prior art power supplies without the power losses associated with such prior art power supplies.
There are a variety of fault conditions to which lighting controls may be subject, including, for example, over voltage and over current conditions. Over voltage conditions can be caused by, for example, the turning on and off of nearby and connected magnetic loads, capacitive coupling to parallel wire runs with sharp transient loads, lightning strikes, etc. Over current conditions can be caused by, for example, short circuited loads, connected loads exceeding the control's rating, mis-wire conditions, etc. Semiconductor devices, such as MOSFETs, have limits as to how much voltage and current they can withstand without failure. In order to protect a control that uses these semiconductor devices from failure, these limits are preferably never exceeded. Fast detection of fault conditions, and fast reaction thereto is desirable in order to protect these devices.
In contrast, during normal operation, the rates of transition between conductive and non-conductive states of these semiconductor devices are controlled to be slow. These slow rates of transition are used, for example, to limit the voltage and current waveforms as seen by the load, to comply with radiated and conducted radio frequency interference (RFI) limits, or to limit voltage ringing caused by inductive power wiring. However, these slow rates of transition during normal operation are too slow for adequate protection of these semiconductor devices. Thus, there is a need for protection circuitry that operates to cause fast rates of transition under fault conditions, while still allowing these semiconductor devices to be operated with slow rates of transition under normal operating conditions.